UDP(4)                 FreeBSD Kernel Interfaces Manual                 UDP(4)

NAME
     udp - Internet User Datagram Protocol

SYNOPSIS
     #include <sys/types.h>
     #include <sys/socket.h>
     #include <netinet/in.h>

     int
     socket(AF_INET, SOCK_DGRAM, 0);

DESCRIPTION
     UDP is a simple, unreliable datagram protocol which is used to support
     the SOCK_DGRAM abstraction for the Internet protocol family.  UDP sockets
     are connectionless, and are normally used with the sendto(2) and
     recvfrom(2) calls, though the connect(2) call may also be used to fix the
     destination for future packets (in which case the recv(2) or read(2) and
     send(2) or write(2) system calls may be used).

     UDP address formats are identical to those used by TCP.  In particular
     UDP provides a port identifier in addition to the normal Internet address
     format.  Note that the UDP port space is separate from the TCP port space
     (i.e., a UDP port may not be "connected" to a TCP port).  In addition
     broadcast packets may be sent (assuming the underlying network supports
     this) by using a reserved "broadcast address"; this address is network
     interface dependent.

     Options at the IP transport level may be used with UDP; see ip(4).
     UDP_ENCAP socket option may be used at the IPPROTO_UDP level to
     encapsulate ESP packets in UDP.  Only one value is supported for this
     option: UDP_ENCAP_ESPINUDP from RFC 3948, defined in <netinet/udp.h>.

MIB VARIABLES
     The udp protocol implements a number of variables in the net.inet branch
     of the sysctl(3) MIB.

     UDPCTL_CHECKSUM    (udp.checksum) Enable udp checksums (enabled by
                        default).

     UDPCTL_MAXDGRAM    (udp.maxdgram) Maximum outgoing UDP datagram size

     UDPCTL_RECVSPACE   (udp.recvspace) Maximum space for incoming UDP
                        datagrams

     udp.log_in_vain    For all udp datagrams, to ports on which there is no
                        socket listening, log the connection attempt (disabled
                        by default).

     udp.blackhole      When a datagram is received on a port where there is
                        no socket listening, do not return an ICMP port
                        unreachable message.  (Disabled by default.  See
                        blackhole(4).)

ERRORS
     A socket operation may fail with one of the following errors returned:

     [EISCONN]          when trying to establish a connection on a socket
                        which already has one, or when trying to send a
                        datagram with the destination address specified and
                        the socket is already connected;

     [ENOTCONN]         when trying to send a datagram, but no destination
                        address is specified, and the socket has not been
                        connected;

     [ENOBUFS]          when the system runs out of memory for an internal
                        data structure;

     [EADDRINUSE]       when an attempt is made to create a socket with a port
                        which has already been allocated;

     [EADDRNOTAVAIL]    when an attempt is made to create a socket with a
                        network address for which no network interface exists.

SEE ALSO
     getsockopt(2), recv(2), send(2), socket(2), blackhole(4), inet(4),
     intro(4), ip(4), udplite(4)

HISTORY
     The udp protocol appeared in 4.2BSD.

FreeBSD 13.1-RELEASE-p6        February 6, 2017        FreeBSD 13.1-RELEASE-p6